Plasma-assisted processing apparatus

ABSTRACT

A plasma-assisted processing apparatus comprises: a vacuum vessel defining a processing chamber, a gas supply line for carrying gases into the processing chamber, a workpiece support for supporting a workpiece, disposed in the processing chamber and serving as an electrode, a disk antenna for radiating a high-frequency wave of a frequency in the VHF or the UHF band into the processing chamber, a high-frequency waveguide for guiding a high-frequency wave to the disk antenna, and a window of a dielectric material isolating the disk antenna from the processing chamber. A conductive ring is disposed between the disk antenna and the window such that its end surface is in contact with a peripheral part of the disk antenna.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a plasma-assisted processingapparatus and, more particularly, to a plasma-assisted processingapparatus capable of producing a highly dense, highly uniform plasmaunder different conditions defined by various parameters including thetypes of gases, the pressure of gases and high-frequency power, whichare variable in wide ranges, and of satisfactorily processing aworkpiece by a plasma-assisted process.

[0002] Miniaturization of the components of ultralarge-scale integratedcircuits (ULSI circuits) has made a rapid progress in recent years andULSI circuits of minute structure having a minimum feature length on theorder of 0.13 μm have been developed. Semiconductor wafers having alarge diameter on the order of 300 mm have been used for forming suchULSI circuits thereon. There has been a need for processing apparatusescapable of accurately etching workpieces to form DRAMs and flashmemories as well as system LSIs, and of processing large-diametersemiconductor wafers.

[0003] To meet such requirement, a plasma-assisted process not onlycapable of highly uniformly processing a large area but also having anadvanced control characteristic is necessary. A plasma-assistedprocessing apparatus must be provided with a plasma-assisted processingunit capable of fine processing, and standards for dimensions havebecome strict. For example, a plasma-assisted etching process mustprevent the occurrence of shape anomaly called “notch” resulting fromthe accumulation of positive charges in the bottom of a minute pattern.Negative gases used for etching, such as Cl₂, BCl₃, SF₆ and such,produce negative ions during an etching process. Those negative ionshave a function to neutralize positive charges accumulated in the bottomof a minute pattern. Since negative ions are produced more easily atlower electron temperatures, it is desired to realize a plasma of a lowelectron temperature. Such a plasma of a low electron temperature can beproduced by a plasma-assisted processing apparatus using high-frequencypower at a frequency in the VHF or the UHF band.

[0004] In a plasma-assisted processing apparatus, a plasma is producedthrough the capacitive coupling of an antenna or a counter electrode,when the frequency of high-frequency power applied to theplasma-assisted processing apparatus is 10 MHz or below. The wavelengthof the high-frequency power is far smaller than the diameter of anantenna and any potential distribution is not formed on the antenna.Therefore, a uniform plasma is produced in front of the antenna.

[0005] When the frequency of the high-frequency power applied to theplasma-assisted processing apparatus is not lower than those in the VHFband, the wavelength of the high-frequency power is short and is long ascompared with the diameter of the antenna. Consequently, the uniformityof the plasma produced in front of the antenna is unsatisfactory.

[0006]FIG. 12 is a schematic sectional view of an essential part of aknown plasma-assisted processing apparatus using a high-frequency powerat a frequency in the VHF or the UHF band.

[0007] Shown in FIG. 12 are a case 50, a vacuum vessel 51, a processingchamber 52, a workpiece support (electrode) 53, a workpiece (wafer) 54,a gas supply passage 55, an exhaust passage 56, a first high-frequencypower source 57, a high-frequency waveguide 58, a matching device, ashield 60, a disk antenna 61, a dielectric material 62, magnetic fieldcreating parts 63, a window 64, a gas-diffusing plate 65 and a secondhigh-frequency power source 66.

[0008] The vacuum vessel 51 is placed in the case 50. The vacuum vessel51 defines the processing chamber 52. The exhaust passage 56 isconnected for evacuation to a lower part of the vacuum vessel 51. Theworkpiece support 53 supporting the workpiece 54 is placed in theprocessing chamber 52. A large open end of the vacuum vessel 51 isclosed hermetically by the window 64 and the gas-diffusing plate 65. Thegas supply passage 55 is connected to the gas-diffusing plate 65 tosupply gases through the gas-diffusing plate 65 into the processingchamber 52. The disk antenna 61 is placed on the window 64. The diskantenna 61 and the dielectric material 62 are covered with the shield60. The high-frequency waveguide 58 penetrates the shield 60, extendsthrough a through hole formed in the case 50 to connect the disk antenna61 to the external first high-frequency power source 57. Thehigh-frequency waveguide 58 has one end joined to the disk antenna 61and the other end connected through the matching device 59 to the firsthigh-frequency power source 57. The high-frequency waveguide 58 guides ahigh-frequency power at a frequency in the UHF band (or the VHF band)generated by the first high-frequency power source 57 to the diskantenna 61. The magnetic field creating parts 63 are disposed in thecase 50 to create a magnetic field in the processing chamber 52. Thesecond high-frequency power source 66 is connected to the workpiecesupport 53 to supply high-frequency power at a frequency in the UHF band(or the VHF band) to the workpiece support 53.

[0009] When processing the workpiece 54 by the plasma-assistedprocessing apparatus, gases are supplied through the gas supply passage55 into the processing chamber 52, the first high-frequency power source57 applies the high-frequency power to the disk antenna 61, the secondhigh-frequency power source 66 applies the high-frequency power to theworkpiece support 53, and the magnetic field creating parts 63 creates amagnetic field in the processing chamber 52. Thus, a plasma is producedin the processing chamber 52. The plasma acts on the surface of theworkpiece 54 for plasma-assisted processing.

[0010] Since the frequency of the high-frequency power is in the UHFband (or the VHF band), the high-frequency wave carrying thehigh-frequency power assumes the aspect of an electromagnetic wave. Thishigh-frequency wave propagates only on the boundary region of the plasmaand is absorbed. The high-frequency wave is not radiated simply from thedisk antenna 61, but also forms a standing wave in a sheath region onthe boundary of the plasma and in the high-frequency waveguide 58. Thestrength distribution of an electric field is dependent on the size andshape of the boundary region of the plasma. To create a high-frequencyelectric field of a desired strength distribution, such as a flatdistribution extending in the length (diameter) of the workpiece 54,notice must be taken not only of an electric field created in a regionunder the disk antenna 61, but also of an electric field created aroundthe workpiece 54, because the high-frequency electric field createdaround the workpiece 54 tends to enlarge, the high-frequency power isconcentrated on the region in which the plasma is produced after theplasma has been produced in the region around the workpiece 54 and,consequently, the density of the plasma around the workpiece 54increases progressively.

[0011]FIGS. 13A to 13D are views and graphs, respectively, of assistancein explaining the creation of such a high-frequency electric field. FIG.13A is a fragmentary sectional view of the plasma-assisted processingapparatus, FIG. 13B is a pictorial view showing an strength distributionof an electric field, FIG. 13C is a graph showing the strengthdistribution of an electric field with respect to a direction along thediameter of the disk antenna 61, and FIG. 13D is a graph showing thevariation of power absorption with position with respect to the diameterof the disk antenna 61, in which the frequency f of the high-frequencypower is 450 MHz, and the window 64 is formed of quartz (specificdielectric constant: 3.5)

[0012] In FIG. 13A, indicated at 67 is the sheath region and other partslike or corresponding to those shown in FIG. 12 are denoted by the samereference characters. In FIG. 13C, distance (m) from the center of theworkpiece 54 is measured on the horizontal axis, and the ratio ofelectric field strength E_(edge) at an optional part of the workpiece 54to electric field strength E_(center) at a central part of the workpiece54 is measured on the vertical axis. In FIG. 13D, distance (m) from thecenter of the workpiece 54 is measured on the horizontal axis, and powerof an electromagnetic wave absorbed by the plasma (absorbed power) ismeasured on the vertical axis.

[0013] As shown in FIG. 13B, a high-frequency wave of a frequency in theUHF band propagates through the window 64 and the sheath region(boundary region of the plasma) 64. As shown in FIG. 13C, a strengthdistribution curve indicating the strength distribution of the electricfield created right below the window 64 has a node at a partcorresponding to the distance 110 mm from the center of the workpiece 54(TM₀₁ mode), and a part of the electric field around a peripheral partof the workpiece 54 has an electric field strength E_(edge). Thehigh-frequency wave of the frequency in the UHF band is concentrated ona part where the plasma density increases, further enhancing theconcentration of the high-frequency wave of the frequency in the UHFband on the same part. Consequently, as shown in FIG. 13D, the absorbedpower of the high-frequency wave and the plasma density distributionchange when the high-frequency power (density) is changed.

[0014] In the known plasma-assisted processing apparatus using thehigh-frequency wave of a frequency in the VHF or the UHF band, theuniformity of the plasma density distribution in front of the antenna isdisturbed and the plasma density distribution changes when thehigh-frequency power (density) is changed.

[0015] There are some known plasma-assisted processing apparatus using ahigh-frequency wave of a frequency in the VHF or the UHF band, capableof producing an improved plasma. A first known plasma-assistedprocessing apparatus disclosed in Japanese Patent Laid-open No.2000-195843 is provided with a disk-shaped counter electrode disposedopposite to a wafer, i.e., a workpiece, with a dielectric materialplaced between the counter electrode and the wafer. A second knownplasma-assisted processing apparatus disclosed in Japanese PatentLaid-open No. 7-307200 is provided with a radial antennal structure forradiating a high-frequency wave, formed by alternately arranging aplurality of antenna elements radially extending from the center of theantenna and a plurality of antenna elements extending from the peripherytoward the center of the antenna. A third known plasma-assistedprocessing apparatus disclosed in Japanese Patent Laid-open No. 10-12396is provided with an antenna structure including an inner antennaconductor and an outer antenna conductor, having a length different fromthe inner antenna, disposed at different levels, respectively to form aresonance structure for producing a uniform plasma. A fourth knownplasma-assisted processing apparatus disclosed in Japanese Patentlaid-open no. 2000-195843 is provided with a disk-shaped electrode(antenna) disposed opposite to a wafer, i.e., a workpiece, and providedwith an annular groove as a plasma trap to produce a uniform plasma andto control plasma density distribution on the wafer.

[0016] The first known plasma-assisted processing apparatus is intendedto moderate the potential distribution of the high-frequency wave on thecounter electrode by disposing the dielectric material between theantenna and the counter electrode. However, the production of a plasmaby an electromagnetic wave that propagates along the surface of thecounter electrode is dominant because the plasma is produced by thecapacitive coupling dependent on the potential distribution of thehigh-frequency wave, and effect on the moderation of the potentialdistribution of the high-frequency wave on the counter electrode iscomparatively unsatisfactory.

[0017] The second known plasma-assisted processing apparatus uses theradial antenna structure, in which intervals between the antennaelements increase toward the periphery of the radial antenna structure.Therefore, the electric field strength in a region around the peripheralpart of the radial antenna structure is low, and boundary conditions forthe electromagnetic wave in a region in which the antennal elementexists and in a region in which any antenna element does not exist aredifferent. Therefore, the electric field strength is not fixed withrespect to the circumferential direction on the radial antennastructure.

[0018] In the third known plasma-assisted processing apparatus, anintense electromagnetic wave radiated by the antenna propagates throughthe sheath region when a desired resonance structure is formed.Therefore, the pattern of antenna radiation is different from thepattern of the electric field in the sheath region and hence plasmadensity is not necessarily distributed in a uniform plasma densitydistribution.

[0019] In the fourth known plasma-assisted processing apparatus, theplasma trap formed on the counter electrode is in a plasma-producingregion. Therefore, an electromagnetic wave radiated from the counterelectrode is enhanced by the plasma trap, plasma density around theplasma trap increases, the high-density plasma produced in the regiondiffuses into a region around the workpiece, and a uniform plasma, whichis more uniform than a plasma produced by a plasma-assisted processingapparatus not provided with any plasma trap, is produced around theworkpiece. However, since the plasma flows into the annular grooveserving as the plasma trap, it is difficult to produce a still moreuniform plasma.

[0020] Those known plasma-assisted processing apparatuses using ahigh-frequency wave of a frequency in the VHF or the UHF band havedifficulty in producing a uniform plasma in a region in which theworkpiece is placed, and take nothing into consideration to prevent thevariation of the density of the plasma produced around the workpiecedependent on the variation of the process parameters.

SUMMARY OF THE INVENTION

[0021] The present invention has been made in view of such a technicalbackground and it is therefore an object of the present invention toprovide a plasma-assisted processing apparatus capable of producing ahighly uniform, high-density plasma around the entire workpiece by usinga high-frequency wave of a frequency in the VHF or the UHF bandregardless of the variation of process parameters.

[0022] According to the present invention, a plasma-assisted processingapparatus includes: a vacuum vessel defining a processing chamber; a gassupply line for carrying gases into the processing chamber; a workpiecesupport for supporting a workpiece, disposed in the processing chamberand serving as an electrode; a disk antenna for radiating ahigh-frequency wave of a frequency in the VHF or the UHF band into theprocessing chamber; a high-frequency waveguide for guiding ahigh-frequency wave to the disk antenna; and a window of a dielectricmaterial isolating the disk antenna from the processing chamber; whereina conductive ring is disposed between the disk antenna and the windowsuch that its end surface is in contact with a peripheral part of thedisk antenna.

[0023] The conductive ring is disposed with its end surface in contactwith the peripheral part of the disk antenna to generate a standing wavein a space surrounded by the conductive annular ring. Thus, the strengthof a part of an electric field in the space surrounded by the conductivering is enhanced and the strength of a part of the electric field aroundthe conductive ring decreases relatively. Therefore, the variation ofpower absorbed by the plasma, i.e., the variation of the plasma densitydistribution, can be suppressed even if the high-frequency power(density) varies.

[0024] A second window of a dielectric material may be superposed on thewindow, and the window and the second window may be formed of differentdielectric materials respectively having different dielectric constants,respectively.

[0025] Use of such a means enhances the standing wave in the boundaryregion of the plasma through the enhancement of the high-frequencystanding wave in a region right under the disk antenna, and the functionof the means can be enhanced.

[0026] An antenna height adjusting means capable of moving the diskantenna to adjust the distance between the disk antenna and the windowor to adjust the distance between the disk antenna and the window andthe distance between the disk antenna and the second window may beconnected to the disk antenna.

[0027] Thus, the position of a node in the high-frequency standing waveright under the disk antenna can be moved along the diameter of the diskantenna, whereby the plasma density distribution can be optionallyadjusted to a plasma density distribution compatible with gases and afilm to be processed.

[0028] The conductive ring may be formed in an inside diameter in therange of an integral multiple of half the wavelength of thehigh-frequency wave propagating through the conductive ring ±10%.

[0029] Thus, the high-frequency standing wave right under the diskantenna can be enhanced and hence a plasma can be easily produced rightunder the disk antenna.

[0030] A conductive member having the shape of a rod or a cylinder of aheight equal to that of the conductive ring may be disposed in a centralregion of a space surrounded by the conductive ring and corresponding toa central part of the disk antenna.

[0031] Thus, the strength of a part of the high-frequency electric fieldcorresponding to the central part of the disk antenna can be enhanced,whereby processing speed at which a central part of the workpiece isprocessed can be increased.

[0032] A dielectric ring or cylinder of a height nearly equal to that ofthe conductive ring may be disposed in central region of a spacesurrounded by the conductive ring and corresponding to a central part ofthe window.

[0033] Thus, the reduction of the strength of a part of thehigh-frequency electric field corresponding to a central part of thedisk antenna can be avoided, whereby a uniform electric field can becreated around the central part of the disk antenna.

[0034] The disk antenna and the high-frequency waveguide may be formedin dimensions meeting an inequality: a/R_(d)≦0.4 or a/R_(d)≧0.6, where ais the radius of the disk antenna, and R_(d) is the effective radius ofthe high-frequency waveguide.

[0035] Thus, the strength of a part of the high-frequency electric fieldaround the disk antenna can be reduced and the density of a part of theplasma around the disk antenna can be reduced, where by a highlyuniform, high-density plasma can be stably produced around the entireworkpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a schematic sectional view of an essential part of aplasma-assisted processing apparatus in a first embodiment according tothe present invention;

[0037]FIG. 2 is a schematic sectional view of a disk antenna andassociated parts of a plasma-assisted processing apparatus in a secondembodiment according to the present invention;

[0038]FIG. 3 is a schematic sectional view of an essential part of aplasma-assisted processing apparatus in a third embodiment according tothe present invention;

[0039]FIG. 4 is a schematic sectional view of a disk antenna andassociated parts of a plasma-assisted processing apparatus in a fourthembodiment according to the present invention;

[0040]FIG. 5 is a schematic sectional view of a disk antenna andassociated parts of a plasma-assisted processing apparatus in a fifthembodiment according to the present invention;

[0041]FIG. 6 is a schematic sectional view of a disk antenna andassociated parts of a plasma-assisted processing apparatus in a sixthembodiment according to the present invention;

[0042]FIG. 7 is a schematic sectional view of a disk antenna andassociated parts of a plasma-assisted processing apparatus in a seventhembodiment according to the present invention;

[0043]FIGS. 8A and 8B are graphs showing an electric field strengthdistribution and an absorbed power distribution, respectively, in theplasma-assisted processing apparatus in the first embodiment;

[0044]FIGS. 9A, 9B and 9C are a sectional view and graphs of assistancein explaining electric field strength distribution and absorbed powerdistribution in the plasma-assisted processing apparatus in the secondembodiment;

[0045]FIG. 10 is a graph showing the electric field strengthdistribution with respect to a direction along the diameter of the diskantenna of the plasma-assisted processing apparatus in the fifthembodiment;

[0046]FIG. 11 is a graph showing the variation of the strength of ahigh-frequency electric field when the ratio of the radius of the diskantenna to the effective diameter of a waveguide included in theplasma-assisted processing apparatus in the seventh embodiment ischanged;

[0047]FIG. 12 is a schematic sectional view of an essential part of aknown plasma-assisted processing apparatus using a high-frequency powerat a frequency in the VHF or the UHF band; and

[0048]FIGS. 13A, 13B, 13C and 13D are views of assistance in explainingconditions of high-frequency electric fields.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] Preferred embodiments of the present invention will be describedwith reference to the accompanying drawings.

[0050] Referring to FIG. 1 showing a plasma-assisted processingapparatus in a first embodiment according to the present invention,there are shown a vacuum vessel 1, a processing chamber 2 defined by thevacuum vessel 1, a workpiece support 3 (support electrode), a workpiece4, a gas supply passage 5, an exhaust passage 6, a first high-frequencypower source 7, a high-frequency waveguide 8, a matching device 9, ashield 10, a disk antenna 11, a dielectric material 12, a conductivering 13, a window 14, a gas-diffusing plate 15, a second high-frequencypower source 16, a filler 17, magnetic field creating parts 18 and acase 19.

[0051] The vacuum vessel 1 defining the processing chamber 2 is placedin the case 19. The exhaust passage 6 is connected for evacuation to alower part of the vacuum vessel 1. The workpiece support 3 is placed inthe processing chamber 2 and the workpiece 4 is mounted on the workpiecesupport 3. The open upper end of the vacuum vessel 1 is closedhermetically by the window 14, and the gas-diffusing plate 15 is placedcontiguously with the lower surface of the window 14. The gas supplypassage 5 is connected to the gas-diffusing plate 15. Gases are suppliedthrough the gas-diffusing plate 15 into the processing chamber 2. Thedisk antenna 11 is put on a conductive ring 13 placed on the window 14.The disk antenna 11 is surrounded by the dielectric material 12. Thedisk antenna 11 and the dielectric material 12 are covered with theshield 10. The conductive ring 13 has one end surface in contact with aperipheral part of the disk antenna 11 and the other end surface incontact with the window 14. A space surrounded by the conductive ring 13is filled up with the filler 17. The high-frequency waveguide 8 extendsthrough the shield 10 and the case 19 to connect the disk antenna 11 tothe external first high-frequency power source 7. The high-frequencywaveguide 8 has one end joined to a central part of the disk antenna 11and the other end connected through the matching device 9 to the firsthigh-frequency power source 7. The first high-frequency power source 7generates a high-frequency wave of a frequency in the UHF band (or theVHF band). The high-frequency wave is applied through the high-frequencywaveguide 8 to the disk antenna 11. The magnetic field creating parts 18are disposed inside the case 19 to create a magnetic field in theprocessing chamber 2. The external second high-frequency power source 16is connected to the workpiece support 3 to apply a high-frequency waveof a frequency in the UHF band (or the VHF band) to the workpiecesupport 3.

[0052] Basic operations of the plasma-assisted processing apparatus inthe first embodiment are substantially the same as those of the knownplasma-assisted processing apparatus shown in FIG. 12. Process gases aresupplied through the gas supply passage 5 and the gas-diffusing plate 15into the processing chamber 2. A high-frequency wave of a frequency inthe UHF band (or the VHF band), such as 450 MHz, generated by the firsthigh-frequency power source 7 is applied through the high-frequencywaveguide 8 and the matching device 9 to the disk antenna 11 to create ahigh-frequency electric field under the disk antenna 11 and, at the sametime, a high-frequency wave of a frequency in the UHF band (or the VHFband), such as 450 MHz, generated by the second high-frequency powersource 16 is applied to the workpiece support 3 and the magnetic fieldcreating parts 18 creates a magnetic field in the processing chamber 2.Consequently, a plasma is produced in the processing chamber 2. Theplasma is applied to the workpiece 4 to process the surface thereof.

[0053] Since the conductive ring 13 is disposed with its end surface incontact with the peripheral part of the disk antenna 11, ahigh-frequency standing wave is generated in a space surrounded by theconductive ring 13 to enhance the strength of a part of the electricfield in the space surrounded by the conductive ring 13 and to reducethe strength of a part of the electric field around the conductive ring13 relatively.

[0054]FIGS. 8A and 8B are graphs showing an electric field strengthdistribution and an absorbed power distribution, respectively, in theplasma-assisted processing apparatus in the first embodiment.

[0055] In FIG. 8A, distance (m) from the center of the disk antenna 11is measured on the horizontal axis, and the ratio E₁/E₀, where E₀(E_(center)) is the electric field strength of a part of the electricfield around the center of the disk antenna 11 and E₁ (E_(edge)) is theelectric field strength of a part of the electric field around a part ofthe disk antenna 11 at a distance from the center of the disk antenna11, i.e., values of E₁ normalized by E₀, is measured on the verticalaxis. In FIG. 8B, distance (m) from the center of the disk antenna 11 ismeasured on the horizontal axis, and absorbed power absorbed by theplasma from the high-frequency wave is measured on the vertical axis.

[0056] As obvious from FIG. 8A, the electric field has a maximumelectric field strength around the center of the disk antenna 11 and theelectric field strength decreases with distance from the center of thedisk antenna 11. The electric field strength is substantially zero atthe circumference of the disk antenna 11. As obvious from FIG. 8B,absorbed power is comparatively large around the center of the diskantenna 11, decreases gradually toward the circumference of the diskantenna 11. Absorbed power is substantially zero in a region around thecircumference of the disk antenna 11. Absorbed power absorbed by thepart of the plasma around a peripheral part of the disk antenna 11 ofthe plasma-assisted processing apparatus in the first embodiment is farsmaller than that shown in FIG. 13D absorbed by the part of the plasmaaround the disk antenna of the known plasma-assisted processingapparatus shown in FIG. 12. Therefore, the range of variation ofabsorbed power, i.e., the range of variation of plasma densitydistribution, in the plasma-assisted processing apparatus in the firstembodiment is narrow even if high-frequency power (density) changes andthe variation of plasma density distribution dependent on processparameters can be reduced.

[0057]FIG. 2 shows a disk antenna 11 and associated parts of aplasma-assisted processing apparatus in a second embodiment according tothe present invention, in which parts like or corresponding to those ofthe plasma-assisted processing apparatus in the first embodiment aredenoted by the same reference characters. The plasma-assisted processingapparatus in the second embodiment is provided with a second window 20.

[0058] Referring to FIG. 2, the second window 20 is sandwiched between awindow 14 and a conductive ring 13. The second window 20 is formed of adielectric material having a dielectric constant different from that ofa dielectric material forming the window 14. The plasma-assistedprocessing apparatus in the second embodiment is the same in otherrespects as the plasma-assisted processing apparatus in the firstembodiment. A void may be formed instead of the second window 20 of thedielectric material.

[0059] If a void is formed instead of the second window 20, the voidexercises two functions, i.e., a first function to create a strongelectric field under the disk antenna 11 and a second function to reducethe effective dielectric constant of the window 14 as judged from theside of a plasma.

[0060] As mentioned above, a high-frequency wave of a frequency in theUHF band (or the VHF band) propagates through a sheath region formed onthe boundary of a plasma. The specific dielectric constantε=1-n_(es)e²/ε₀m_(e)ω²≦1, where n_(es) is plasma density in the sheathregion, e is elementary electric charge, ε₀ is the dielectric constantof a vacuum space, m_(e) is the mass of an electron, and ω is theangular frequency of the high-frequency wave. Thus, a three-layerstructure consisting of the second window 20 (ε=1) of air or vacuum, thewindow 14 of quartz (ε=3.5) and the sheath region (ε<1) is formedbetween the disk antenna 11 and the boundary of the plasma. Thisthree-layer structure enhances the strength of a high-frequency standingwave generated right under the disk antenna 11, the strength of astanding wave on the boundary of the plasma is enhanced and plasmadensity of a part of the plasma under the disk antenna 11 can beincreased.

[0061] In this state, the wavelength λ of the high-frequency wave in aspace surrounded by the conductive ring 13 and filled up with adielectric material 12 is expressed by: λ=λ₀/ε_(r) ^(½), where λ₀ is thewavelength of the high-frequency wave in a vacuum and ε_(r) is thedielectric constant of the dielectric material 12. The dielectricconstant ε of the combination of the windows 14 and 20 is expressed by:ε=ε_(r1) ^(½)×{d₁/(ε_(rl) ^(½)d₁+ε_(r2) ^(½)d₂)}×ε_(r1), where d₁ is thethickness of the window 14, ε_(r1) is the dielectric constant of thewindow 14, d₂ is the thickness of the void or the second window 20, andε_(r2) is the dielectric constant of the void or the second window 20.

[0062]FIGS. 9A, 9B and 9C are a sectional view and graphs of assistancein explaining the condition of the electric field created in theplasma-assisted processing apparatus in the second embodiment. FIG. 9Ais a fragmentary, schematic sectional view of the plasma-assistedprocessing apparatus, FIG. 9B is a view of assistance in explaining thecondition of the electric field in the sheath region, dependent on thethickness d₂ of the void or the second window 20, and FIG. 9C is a graphshowing the variation of electric field strength dependent on thethickness d₂ of the void or the second window 20 with distance along thediameter of the disk antenna 11.

[0063] In FIG. 9A, indicated at 2′ is the sheath region, and parts likeor corresponding to those shown in FIG. 2 are denoted by the samereference characters.

[0064] In FIG. 9B, distance (m) from the center of the disk antenna 11is measured on the horizontal axis, and the ratio |E|/|E_(center)|,where |E_(center)| is the electric field strength of a part of theelectric field around the center of the disk antenna 11 and |E| is theelectric field strength of a part of the electric field around a part ofthe disk antenna 11 at a distance from the center of the disk antenna11, is measured on the vertical axis, E_(r) is a component of theelectric field in the direction of diameter and E_(z) is a component ofthe electric field in the direction of height. In FIG. 9B, an uppergraph shows electric field strength distribution when d₂=0, i.e., whenthe plasma-assisted processing apparatus does not have the void or thesecond window, and a lower graph shows electric field strengthdistribution when d₂=20, i.e., when the plasma-assisted processingapparatus has the second window 20 of 20 mm in thickness.

[0065] In FIG. 9C, distance (m) from the center of the disk antenna 11is measured on the horizontal axis, and |E| (normalized), i.e., valuesof electric field strength E₁ (E_(edge)) at parts at distances from thecenter of the disk antenna 11 normalized by the electric field strengthE₀ at the center of the disk antenna 11, is measured on the verticalaxis. In FIG. 9C, curves indicate distributions of |E| for cases whenthe plasma-assisted processing apparatus does not have the void or thesecond window (d₂=0), and where the thickness of the void or the secondwindow 20 is 10 mm (d₂=20), 20 mm (d₂=20) and 30 mm (d₂=30),respectively.

[0066] The components E_(r) and E_(z) shown in FIG. 9B are dependent onJ₁ (β_(r)) and J₀ (β_(r)), respectively, where β is wavelength in theplasma, J₀ is zero-order Bessel function and J₁ is first-order Besselfunction, and are the same as the TM₀₁ mode. When the frequency of thehigh-frequency wave is 450 MHz, the quarter wavelength of thehigh-frequency wave in a vacuum is 166 mm and that of the same in quartzis 88 mm. In this case, as sown in FIG. 9B, the node of the componentE_(z) shifts outward as the thickness d₂ of the void or the secondwindow 20 increases and the effective wavelength of the high-frequencywave increases. For example, suppose that the frequency of thehigh-frequency wave is 450 MHz and the thickness d₂ of the second window20 is 35 mm. Whereas the electric field strength at a part where r=150mm is low when the plasma-assisted processing apparatus is provided withneither the void nor the second window 20 (d₂=0), the |E| (normalized)remains comparatively constant when the plasma-assisted processingapparatus is provided with the second window 20 of d₂≧10 mm as shown inFIG. 9C.

[0067] In the foregoing examples, the respective specific dielectricconstants ε_(r1) and ε_(r2) of the window 14 and the second window 20meet an inequality: ε_(r1)>ε_(r2). However, this relation between thespecific dielectric constants ε_(r1) and ε_(r2) is dependent on thefrequency of the high-frequency wave used and the respective specificdielectric constants ε_(r1) and ε_(r2) of the window 14 and the secondwindow 20. Therefore, this relation is generalized as follows.

[0068] The specific dielectric constants ε_(r1) and ε_(r2) of the window14 and the second window 20 and the thickness d₂ of the second window 20are expressed by a general expression using, as parameters, thewavelength of the high-frequency wave to be used and the radius R of theworkpiece 4 to be processed. The proper radius R of the workpiece 4 maybe about ¼ of the effective wavelength of the high-frequency wave in thesheath region 2′.

{1−({fraction (1/10)})}R<(λ₀/4/ε_(r1) ^(½))×{ε_(r1) ^({fraction (l/2)})×d ₁/(ε_(r1) ^(½) d ₁+ε_(2r) ^(½) d ₂)}<{1+({fraction (1/10)})}R

[0069] For example, to produce a uniform plasma in a region of 300 mm indiameter by using a high-frequency wave of a frequency in the VHF bandhaving a wavelength longer than those of high-frequency waves in the UHFband and the window 14 of quartz having a specific dielectric constantof 3.5, the second window 20 may be formed of alumina having a largespecific dielectric constant of 9.6.

[0070]FIG. 3 is a schematic sectional view of an essential partincluding a disk antenna 11 of a plasma-assisted processing apparatus ina third embodiment according to the present invention, in which partslike or corresponding to those shown in FIG. 1 are denoted by the samereference characters. Shown in FIG. 3 are a void 21 and antenna heightadjusting members 22.

[0071] The plasma-assisted processing apparatus in the third embodimenthas the void 21 formed between a window 14 and a conductive ring 13,which are the same as those of the plasma-assisted processing apparatusin the first embodiment. The antenna height adjusting members 22 areextended through a shield 10 and are engaged with a disk antenna 11. Theplasma-assisted processing apparatus in the third embodiment isidentical in other respects with the plasma-assisted processingapparatus in the first embodiment.

[0072] The plasma-assisted processing apparatus in the third embodimentis provided with the antenna height adjusting members 22 screwed throughthe shield 10. The antenna height adjusting members 22 are turned tomove the disk antenna 11 vertically. Thus, height of the disk antenna 11and the conductive ring 13 from the window 14 is changed to form thevoid 21 of a desired thickness. Thus, the thickness of the void 21 isadjusted by turning the antenna height adjusting members 22 to move thedisk antenna 11.

[0073] As mentioned above in connection with the description of thesecond embodiment, the effective dielectric constant of the window 14 asjudged from the side of the plasma decreases with the increase of thethickness of the void 21 and, consequently, a node in the electric fieldstrength distribution of the high-frequency electric field created onthe boundary of a plasma shifts toward the circumference. Therefore, theplasma density of a peripheral part of a plasma can be increased byshifting the disk antenna 11 to increase the thickness of the gap 21,and can be decreased or should have a projected distribution in thedirection of diameter by shifting the disk antenna 11 to decrease thethickness of the gap 21.

[0074] The function of an external magnetic field B created by magneticfield creating parts 18 will be described.

[0075] The density p of power supplied to the plasma by thehigh-frequency wave, which will be called “absorbed power”, is expressedby p=σEE*, where σ is conductivity, and E* is the conjugate complexvector of E, and p˜|E|²+α|E×B|. A high-frequency wave of a frequency inthe UHF band propagates in the TM₀₁ mode and does not have any angularcomponent θ when expressed on a cylindrical coordinate system and henceE=(E_(r), 0, E_(z)). When the external magnetic field B is expressed by:B=(B_(r), 0, B_(z)) and the plasma density in a central part (r=0) canbe increased by using a magnetic field strength distribution having ahigh B_(r), and the plasma density in a middle part can be increased byusing a magnetic field strength distribution having a high B_(z) at apeak of E_(r).

[0076] In the plasma-assisted processing apparatus in the thirdembodiment, the position of the disk antenna 11 is adjusted by theantenna height adjusting members 22 so that a desired electric fieldstrength distribution can be achieved. A convex, flat or concave plasmadensity distribution can be optionally achieved by adjusting theintensity and field strength distribution of the external magnetic fieldB to change |E×B|. These adjusting functions are capable of realizingplasma density distributions respectively suitable for various types ofgases and various types of films.

[0077]FIG. 4 is a schematic sectional view of an essential partincluding a disk antenna 11 of a plasma-assisted processing apparatus ina fourth embodiment according to the present invention, in which partslike or corresponding to those shown in FIG. 2 are denoted by the samereference characters. In FIG. 4, indicated at 13R is the inside diameterof a conductive ring 13.

[0078] The inside diameter 13R of the conductive ring 13 of theplasma-assisted processing apparatus in the fourth embodiment is aboutequal to an integral multiple of half the wavelength of a high-frequencywave in a filler 17. The plasma-assisted processing apparatus in thefourth embodiment is identical in other respects with theplasma-assisted processing apparatus in the second embodiment.

[0079] Since the inside diameter 13R of the conductive ring 13 of theplasma-assisted processing apparatus in the fourth embodiment is aboutequal to an integral multiple of half the wavelength of thehigh-frequency wave in the filler 17, the plasma density of a part of aplasma in a region under the disk antenna 11 can be made greater thanthat of part of the plasma in other regions by making the strength of ahigh-frequency standing wave formed in a region surrounded by theconductive ring 13 greater than that of a high-frequency standing waveformed in other regions.

[0080]FIG. 5 is a schematic sectional view of an essential partincluding a disk antenna 11 of a plasma-assisted processing apparatus ina fifth embodiment according to the present invention, in which partslike or corresponding to those shown in FIG. 1 are denoted by the samereference characters. In FIG. 5, indicated at 23 is a conductive memberhaving the shape of a rod or a cylinder.

[0081] The plasma-assisted processing apparatus in the fifth embodimentis provided with a conductive member 23 having the shape of a rod or acylinder and a height equal to that of a conductive ring 13 at aposition corresponding to a central part of the disk antenna. Theplasma-assisted processing apparatus in the fifth embodiment isidentical in other respects with the plasma-assisted processingapparatus in the first embodiment.

[0082]FIG. 10 is a graph showing characteristic curves representingelectric field strength distributions with respect to a direction alongthe diameter of the disk antenna 11 for different heights of theconductive member 23.

[0083] In FIG. 10, distance (m) from the center of the disk antenna 11is measured on the horizontal axis, and the ratio|E_(edge)|/|E_(center)|, where |E_(center)| is the electric fieldstrength of a part of an electric field around the central part of thedisk antenna 11 and |E_(edge)| is the electric field strength of anoptional part of the electric field, is measured on the vertical axis.In FIG. 10, the characteristic curves are for height h₀=0, height h₁₆=16mm, height h₂₄=24 mm, height h_(26.5)=26.5 mm and height h_(34.5)=34.5mm, respectively.

[0084] In the plasma-assisted processing apparatus in the fifthembodiment, the conductive member 23 is placed at a positioncorresponding to the central part of the disk antenna 11 in a spacesurrounded by the conductive ring 13. The electric field strengthdistribution is represented by different characteristics curves fordifferent values of the height of the conductive member 23 as shown inFIG. 10. As obvious from FIG. 10, the electric field strength at thecenter of the disk antenna 11 when the conductive member 23 is used ishigher that when the conductive member 23 is not used, and the electricfield strength at the center of the disk antenna 11 increases with theincrease of the height of the conductive member 23. For example, whenthe plasma-assisted processing apparatus is used for a process in whichgases which are difficult to dissociate or ionize, such as BCl₃, areused and a large amount of reaction products is produced, such as anetching process for etching a metal, the plasma density of a part of aplasma under a central part of the disk antenna 11 decreases and acentral part of a workpiece is liable to sink. Processing rate at whichthe central part of the workpiece is processed can be adjusted and thecentral part can be properly processed when is used the conductivemember 23 of an appropriate height selected at the central part of thedisk antenna 11 as in the fifth embodiment. The electric field strengthof a part of the electric field corresponding to the central part of thedisk antenna 11 can be enhanced by using the conductive member 23extending into a second window 20 or through the second window 20further into the window 14.

[0085]FIG. 6 is a schematic sectional view of an essential partincluding a disk antenna 11 of a plasma-assisted processing apparatus ina sixth embodiment according to the present invention, in which partslike or corresponding to those shown in FIG. 2 are denoted by the samereference characters. In FIG. 6, indicated at 24 is a dielectric memberhaving the shape of a ring or a cylinder.

[0086] In the plasma-assisted processing apparatus in the sixthembodiment, a space surrounded by a conductive ring 13 is void and isnot filled up with the filler 17 used in the second embodiment, and thedielectric member 24 is placed at a position corresponding to a centralpart of the disk antenna 11 on a second window 20 in the void surroundedby the conductive ring 13. The plasma-assisted processing apparatus inthe sixth embodiment is identical in other respects with theplasma-assisted processing apparatus in the second embodiment.

[0087] The plasma-assisted processing apparatus in the sixth embodimentis provided with the dielectric member 24 having the shape of a ring ora cylinder and disposed at a position corresponding to a central part ofthe disk antenna 11 on the second window 20 in the void surrounded bythe conductive ring 13. Although it is desirable that the dielectricmember 24 has a big height, the dielectric member 24 does not need to beas high as to be in contact with the disk antenna 11.

[0088] The radius d_(s) of the dielectric member 24 having the shape ofa ring or a cylinder may be about equal to half a position correspondingto a peak of the diametrical electric field component E_(r) of ahigh-frequency wave, i.e., about equal to half a quarter of thewavelength of the high-frequency wave in a sheath region 2′. Morespecifically, the approximate value of the radius d_(s) may becalculated by:

d _(s)=ε_(s) ^(½)/(1+ε_(s) ^(½))λ₀/4/2

[0089] where λ₀ is the wavelength of the high-frequency wave in avacuum, and E_(s) is the dielectric constant of the material of thewindow 14. A part of the electric field strength distributioncorresponding to a central part of the disk antenna 11 can beeffectively prevented from sinking when the dielectric member 24 has theshape of a ring. Desirably, the dielectric constant of the material ofthe dielectric member 24 is greater than that of the material of thewindow 14. When the frequency of the high-frequency wave used is 450 MHzand the window 14 is formed of quartz having a dielectric constantε=3.5, alumina is a material suitable for forming the dielectric member24, and the radius d_(s) may be about 54 mm.

[0090]FIG. 7 is a schematic sectional view of an essential partincluding a disk antenna 11 of a plasma-assisted processing apparatus ina seventh embodiment according to the present invention, in which partslike or corresponding to those shown in FIG. 2 are denoted by the samereference characters. In FIG. 7, indicated at 10R is the inside radius(radius of a waveguide) of a shield 10, and at 11R is the radius of thedisk antenna 11.

[0091] The radius 11R=a of the disk antenna 11, the inside radius 10R=bof the shield 10 are selectively determined so that the ratio γ=a/c is0.4 or below or 0.6 or above, where c is the effective diameter of awaveguide expressed by:

[0092] c={a+ε_(s) ^(½)(b−a)}, where ε_(s) is the dielectric constant ofa dielectric material filling up a space between the disk antenna 11 andthe side wall of the shield 10.

[0093]FIG. 11 is a graph showing characteristic curves representing thevariation of high-frequency electric fields with the ratio γ=1/c formaterials filling up the space between the disk antenna 11 and the sidewall of the shield 10.

[0094] In FIG. 11, the ratio γ is measured on the horizontal axis, andthe ratio |E_(edge)|/|E_(center)|, where |E_(center)| is the electricfield strength of a part of an electric field around the central part ofthe disk antenna 11 and |E_(edge)| is the electric field strength of anoptional part of the electric field, is measured on the vertical axis.In FIG. 11, characteristic curves represent the variation of the ratios|E_(edge)|/|E_(center)| for air, alumina and quartz used as thedielectric material filling up the space between the side wall of theshield 10 and the disk antenna 11.

[0095] Description will be given, in connection with the graph shown inFIG. 11, of the effect of the ratio γ=a/c (a is the radius of the diskantenna 11, and c is the effective diameter of the waveguide) on thedetermination of the high-frequency electric field strengthdistribution, and on the determination of the plasma densitydistribution as well.

[0096] The electric field strength in a space around the disk antenna 11changes when the radius a of the disk antenna 11 is changed. As shown inFIG. 11, in which the ratio γ=a/c is measured on the horizontal axis,and the ratio |E_(edge)|/|E_(center)|, is measured on the vertical axis,the electric field strength distribution curves for the differentdielectric materials respectively having different dielectric constantsand filling up the space between the shield 10 and the disk antenna 11are substantially similar. The electric field strength is low in a rangewhere γ≦0.4 and a range where γ≧0.6. When the diameter of the waveguideis 220 mm, a desirable radius a of the disk antenna 11 is 88 mm orbelow, or 132 mm or above. Suppose that the ratio γ* (radius of theantenna)/(effective diameter of the waveguide) is defined by: γ*=γ*(f/f₀), where f₀ is a reference frequency. In this case the referencefrequency is 450 MHz. Then, a range for an index equal to the aforesaidrange applies to the ratio between peripheral electric field strengthand central electric field strength. The diameter of the processingchamber may be used instead of the effective diameter b of thewaveguide.

[0097] Thus, the reduction of the electric field strength in theperiphery of the disk antenna 11 causes the reduction of the plasmadensity in the periphery of the disk antenna 11, which suppresses thechange of the electric field strength distribution in the electric fieldcreated by the high-frequency wave of a frequency in the UHF or the VHFband resulting from the change of the high-frequency power, the pressureof gases or the types of the gases. Consequently, a stable plasma can beproduced always even if the high-frequency power, the pressure of gasesand/or the types of gases are changed.

[0098] The plasma-assisted processing apparatuses in the first to theseventh embodiment enhance the electric field strength of an electricfield corresponding to a region in which the workpiece 4 is placed, forthe high-frequency electric field strength in the UHF or the VHF band,create the electric field of a desired electric field strengthdistribution, such as a flat distribution of a distribution having aslightly sinking middle part, reduce the propagation of thehigh-frequency wave through the peripheral part of the disk antenna 11,and change the effective dielectric constant as judged from the side ofthe plasma by changing the effective distance between the disk antenna11 and the window 14 by the antenna height adjusting members 22, byselectively determining the radius a of the disk antenna 11, disposingthe conductive ring 13 under the peripheral part of the disk antenna 11,and disposing the second window 20 or forming the void between theconductive ring 13 and the window 14 to create a high-frequency electricfield of a desired electric field strength distribution proper for thetypes of gases to be used.

[0099] As apparent from the foregoing description, according to thepresent invention, the conductive ring is disposed with its end surfacein contact with the disk antenna to generate a high-frequency standingwave in the space surrounded by the conductive ring. Therefore, theelectric field strength in the space in which the standing wave isgenerated is increased, and the electric field strength in theperipheral region is reduced relatively. Thus, the variation of theabsorbed power absorbed by the plasma, i.e., the variation of the plasmadensity distribution, can be suppressed even if the high-frequency power(density), the pressure and the type of gases are changed and,consequently, a highly uniform plasma can be produced for processparameters which are subject to change in wide ranges.

What is claimed is:
 1. A plasma-assisted processing apparatuscomprising: a vacuum vessel defining a processing chamber; a gas supplyline for carrying gases into the processing chamber; a workpiece supportfor supporting a workpiece, disposed in the processing chamber andserving as an electrode; a disk antenna for radiating a high-frequencywave of a frequency in the VHF or the UHF band into the processingchamber; a high-frequency waveguide for guiding a high-frequency wave tothe disk antenna; and a window of a dielectric material isolating thedisk antenna from the processing chamber; wherein a conductive ring isdisposed between the disk antenna and the window such that its endsurface is in contact with a peripheral part of the disk antenna.
 2. Theplasma-assisted processing apparatus according to claim 1, wherein asecond window of a dielectric material is superposed on the window, andthe window and the second window are formed of different dielectricmaterials respectively having different dielectric constants,respectively.
 3. The plasma-assisted processing apparatus according to 1or 2, wherein antenna height adjusting means capable of moving the diskantenna to adjust the distance between the disk antenna and the windowor to adjust the distance between the disk antenna and the window andthe distance between the disk antenna and the second window is connectedto the disk antenna.
 4. The plasma-assisted processing apparatusaccording to any one of claims 1 to 3, wherein the conductive ring isformed in an inside diameter in the range of an integral multiple ofhalf the wavelength of the high-frequency wave propagating through theconductive ring ±10%.
 5. The plasma-assisted processing apparatusaccording to any one of claims 1 to 4, wherein a conductive memberhaving the shape of a rod or a cylinder of a height equal to that of theconductive ring is disposed in a central region of a space surrounded bythe conductive ring and corresponding to a central part of the diskantenna.
 6. The plasma-assisted processing apparatus according to anyone of claims 1 to 4, wherein a dielectric ring or cylinder of a heightnearly equal to that of the conductive ring is disposed in a centralregion of a space surrounded by the conductive ring and corresponding toa central part of the window.
 7. The plasma-assisted processingapparatus according to any one of claims 1 to 6, wherein the diskantenna and the high-frequency waveguide are formed in dimensionsmeeting an inequality: a/R_(d)≦0.4 or a/R_(d)≧0.6, where a is a radiusof the disk antenna, and R_(d) is an effective radius of thehigh-frequency waveguide.